This invention relates to the treatment of off-gas from an industrial or hazardous waste treatment system.
Hazardous waste affects human health by its toxicity, deflagrability, corrosivity, reactivity, and infection in addition to being a source of serious pollution. Hazardous wastes have been generally disposed of by land disposal, incineration and recycling. However, as incidents of improper waste disposal, such as emissions of toxic substances from incineration and landfills (e.g. dioxins from incineration and toxic leachate from landfills) begin to create serious health and ecological problems, public awareness has led to increasing legislation and more stringent environmental protection policies. These policies have led to search for other efficient, reliable and cost effective disposal alternatives.
A number of methods based on plasma arc have been proposed to destroy organic and inorganic hazardous waste in all forms, to convert hazardous wastes to a combustible synthetic gas for electricity generation, and to vitrify all the non-combustible materials to a stable glass which can be disposed of safely. However, these methods are considered inefficient and have very high capital and operating costs.
In general, two basic plasma arc technologies, the plasma torch (transferred and non-transferred modes) and the graphite electrode plasma arc (A.C. or D.C.) systems, have been proposed to generate the plasma arc for the hazardous waste destruction or conversion processes.
Systems employing a plasma torch are generally not as energy efficient as those using graphite electrodes due to higher energy loss to the plasma torch cooling water. The efficiency of plasma torches is usually less than 70% especially when the metal plasma torch is placed and operated inside a hot reactor/vessel. Therefore, plasma torches are only effective for gas heating and specialty material processing or fabrication, and they are not practical and economical for material melting. Furthermore, when air is used as the plasma working gas, nitrogen oxides (NOx) and hydrogen cyanide (HCN) are produced due to the reactions of the nitrogen in the air plasma working gas with the oxygen and the hydrocarbons in the vessel/reactor at high temperatures. Furthermore, the steam generated in the vessel will condense on the surface of the metal shroud of the plasma torch. Consequently, carbon black/soot along with the non-dissociated toxic materials will deposit and accumulate on the cold wet metal shroud leading to incomplete destruction of the hazardous wastes. When the plasma torch is removed from the vessel for maintenance, workers are then subject to the exposure to the toxic materials.
The lifetime of the electrodes and the stability (performance) of the plasma arc generated by plasma torches also depend on the atmosphere inside the vessel/reactor. Therefore, the operation of plasma torch systems is more complicated than that of graphite electrode plasma arc systems. Metal plasma torches require high-pressured cooling water for cooling of the internal components. The chemistry and the electrical conductivity of the cooling water must be monitored and adjusted in order to prevent chemical corrosion and mineral depositions inside the torch. Those requirements necessitate expensive auxiliary equipment which increases the capital and operating costs.
Other systems employ graphite electrode electric plasma arc technologies. These systems can lead to either severe oxidation of the graphite electrodes or excessive formation of fine carbon black/soot in the byproduct gas stream. A combined A.C. and D.C. graphite electrodes system was developed to provide electric arc generation and joule resistance heating in the bath simultaneously. Other technologies employ concentric electrodes system and single top D.C. graphite electrode with a conductive bottom for melting and gasification. However, the electrical conductivity of the bottom electrode must be maintained at all times in the single top D.C. graphite electrode system especially when the bottom electrode of the cold vessel/reactor is covered by a layer of slag which is not electrically conductive at low temperatures.
It has been found that the kinetic of carbon black formation was very high during high temperature cracking of hydrocarbons under a slightly reducing condition. Therefore, carbon black/soot is always produced in the reducing plasma arc gasification process and must be removed prior to the downstream air pollution control system. Increasing the residence time of the by-products inside the vessel/reactor or increasing the operating temperature assists in removing carbon black. However, increasing residence time necessitates the use of a larger apparatus or reducing the throughput of feed waste. Accordingly, some systems have been proposed which include an afterburner or thermal oxidizer to increase the reaction kinetics by the turbulent environment as a secondary gas treatment process to ensure complete combustion. However, air and fuel are used in these methods to generate the high heat for the oxidation process. Consequently, a secondary waste stream such as nitrogen oxides may be produced in these systems under such an oxidizing atmosphere.
It would be advantageous to have a system and method for treating the off-gas from waste treatment systems that, at least in part, address these shortcomings.
The present invention provides a system for treating the off-gas from a waste treatment system, such as a graphite electrode arc gasification system, that reduces the carbon black present in the off-gas while avoiding the production of nitrogen oxides and other pollutants. The system includes an afterburner using a plasma torch having a nitrogen-free working gas, which, in one embodiment, is a carbon dioxide and oxygen mixture. The plasma arc ionizes the working gas, thereby creating atomic oxygen, which assists in the removal of carbon black from the off-gas.
In one aspect, the present invention provides an apparatus for treating an off-gas from a waste treatment system. The apparatus includes a refractory-lined cylindrical chamber having an input port for receiving the off-gas and an output port, and a DC powered plasma torch proximate the input port within the chamber, the torch receiving a working gas, the working gas including a mixture of carbon dioxide and oxygen. The plasma torch heats the chamber and the off-gas is thereby converted to an output gas, which is ejected through the output port.
In another aspect, the present invention provides a method for treating an off-gas from a waste treatment system. The method includes the steps of receiving the off-gas at an input port of a refractory-lined cylindrical chamber, heating the chamber by ionizing a working gas using a DC powered plasma torch proximate the input port within the chamber, the working gas including a mixture of carbon dioxide and oxygen, thereby converting the off-gas to an output gas, and outputting an output gas from the chamber.
In a further aspect, the present invention provides a waste treatment system for treating hazardous wastes. The waste treatment system includes a primary waste treatment stage that receives the hazardous waste and produces a by-product off-gas, and a secondary waste treatment stage coupled to the primary waste treatment stage and receiving the off-gas. The secondary waste treatment stage includes a refractory-lined cylindrical chamber having an input port for receiving the off-gas and an output port, and a DC powered plasma torch proximate the input port within the chamber, the torch receiving a working gas, the working gas including a mixture of carbon dioxide and oxygen. The plasma torch heats the chamber and the off-gas is thereby converted to an output gas, which is ejected through the output port.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.